1. Field of the Invention
The present invention relates to an improved rocker assembly for use in engines, and to methods of assembling such improved rocker assemblies.
2. Description of the Prior Art
In piston-powered engines for vehicles and tools such as lawnmowers, the engines have a camshaft and crankshaft working in concert. The pistons move up and down in cylinders, driving the crankshaft. The crankshaft is typically linked to the camshaft, so that the camshaft rotates as the crankshaft rotates. As the camshaft rotates, cylinder valves are opened and allowed to close. As one valve opens, air or a fuel-air mixture is allowed to enter the piston cylinder. As another valve opens, exhaust leaves the piston cylinder.
The camshaft has a plurality of spaced cams that rotate as the camshaft rotates. As the cams rotate, they push against push rods, to reciprocate the push rods. Each push rod has one end that bears against one end of a rocker assembly mounted on the engine. The rocker assemblies include rocker arms that are pivotally mounted on the engine. As each the rocker arm pivots, a second end pushes against a bearing surface on a valve stem to open the cylinder valve. A biasing spring returns the valve to the seated position and the rocker arm pivots back to its original position when the camshaft rotates further.
Commonly, the second end of the rocker arm bearing against the valve stem bearing surface has a wear element for pushing against the valve stem bearing surface. Typically, a slight spacing is maintained between the wear element of the rocker arm and the valve stem bearing surface. This desired spacing--referred to as the valve lash distance--typically varies with the engine manufacturer and end use. For example, in a diesel engine, for an air intake valve, the preferred valve lash distance may be 0.012 inch, while for an exhaust valve, the desired valve lash distance may be 0.018 inch, for example. For a lawnmower engine, the desired valve lash distance may be 0.020 inch for both air intake and exhaust valves. The valve lash distance is set by adjusting a set screw on the first end of the rocker arm to thereby raise or lower the second end of the rocker arm toward or away from the top bearing surface on the valve stem.
As the rocker arm operates, the rocker arm pivots the bearing surface through an arc. Thus, the wear surface of the rocker arm pushes against the bearing surface of the valve stem in a scuffing or scraping motion. Accordingly, the rocker arm wear surface contacts the valve stem bearing surface generally along a line. As the rocker arm wear surface wears, the valve lash distance increases beyond the desirable pre-set distance. This increased distance is generally referred to as excessive back lash. As the back lash increases beyond the desired spacing, engine operation can become noisy as there is additional space for the rocker arm to vibrate. In addition, with greater wear, the timing of the opening and closing of the cylinder valves becomes less exact, resulting in less efficient engine operation. Excessive back lash can also contribute to longevity and environmental problems.
One solution to the excessive back lash problem is to reduce wear by providing a ball and socket on the valve end of the rocker arm. An example of a rocker assembly with such a ball and socket design is shown in FIGS. 1 and 3-5. As shown in FIG. 1, the rocker assembly 10 includes a rocker arm 12 mounted on a base 14 that is mounted on an engine surface 15. A first end 16 of the rocker arm has a bearing element 18 juxtaposed with one end 20 of a push rod 22. The other end 24 of the push rod 22 bears against a cam 26 mounted on a camshaft 28. The camshaft 28 is rotated by a standard drive mechanism, such as a drive chain (not shown) driven by the engine crankshaft (not shown).
The rocker arm 12 also has a second end 30. At the second end 30, a rocker ball 32 is secured to the rocker arm 12. At least a part of the rocker ball 32 is received in a socket 33. The rocker arm 12 is pivotable on the base 14 about an axis 34 between the first end 16 and second end 30 of the rocker arm 12. As the first end 16 of the rocker arm 12 is pushed up by the push rod 22, the rocker arm is pivoted about the axis 34 to move the second end 30 of the rocker arm 12 and the rocker ball 32 through a path defining an arc.
As shown in FIGS. 3-5, the rocker ball 32 has a curved outer surface 36 with a center of curvature 38 and a bottom 40. The center of curvature 38 is between the bottom 40 of the rocker ball 32 and the second end 30 of the rocker arm 12. At least part of the rocker ball defines a portion of a sphere. A non-spherical neck or throat 37 extends up from the spherical portion 39 to an insert 41 received in a pocket in the second end 30 of the rocker arm 12.
As shown in FIGS. 3-5, the socket 33 has an exterior bearing surface 42 for bearing against a complementary valve bearing surface 44 for pushing the valve stem 45. The socket 33 also has an exterior non-bearing surface 46, an interior surface 48 and an annular top 50 with a central opening 52 to receive the rocker ball 32. The interior surface 48 of the socket 33 has a bearing surface 54 for contacting a portion of the rocker ball curved outer surface 36. The interior surface 48 of the socket 33 is curved, although not in a spherical shape. Instead, as in standard ball and socket joints, the interior surface 48 is shaped like a gothic arch, diverging from a low point 56 along a central axis 58. At the annular top 50 of the socket, the horizontal distance between opposing interior sides 60, 62 of the interior surface is greater than the diameter of the spherical part 39 of the rocker ball 32 so that the rocker ball may be inserted into the central opening 52.
To lubricate the interface of the exterior bearing surfaces of the socket and valve element, lubricant channels 70, 72 are machined in the rocker ball 32 and in the socket 33. Through these channels 70, 72, lubricant is delivered to the interface 74 of the socket exterior bearing surface 42 and the valve bearing surface 44.
In operation, the rocker ball and socket assembly have an at rest position, illustrated in FIG. 3. Initially, the rocker ball 32 may be canted slightly to one direction, with the central axis 75 of the rocker ball 32 defining an angle .PHI. with the central axis 58 of the socket 33. The central axis 58 of the socket is parallel to and slightly offset from the axis 77 of the valve stem 45. As the cam 26 pushes up on the push rod 22, the first end 16 of the rocker arm 12 is raised, pivoting the rocker arm 12 about its axis 34. As the rocker arm 12 pivots, the second end 30 of the rocker arm is pushed downward, moving the rocker ball 32 through a curved path. As the rocker ball 32 is thus moved, it pivots in the socket 33. As the rocker arm is pivoted about the axis 34, the rocker ball 32 and socket 33 pass through an interim position, shown in FIG. 4, wherein the central axis 58 of the socket 33 is aligned to be co-linear with the central axis 75 of the rocker ball 32. At the interim position, the socket bearing surface 42 has slid across the valve bearing surface 44, as shown. The axes 58, 75 remain generally parallel with the central axis 77 of the valve stem 45. As the cam 26 pushes the push rod 22 further upward, the first end 16 of the rocker arm 12 is raised higher, and the second end 30 of the rocker arm 12 is pushed further downward, causing further pivoting of the rocker ball 32, and linear and sliding movement of the socket 33. These changes in position are illustrated in FIG. 5, wherein the central axis 75 of the rocker ball is pivoted to define an angle .theta. with the central axis 58 of the socket. The curved outer surface 36 of the rocker ball 32 bears against the bearing surface 54 of the interior surface 48 of the socket, pushing the socket 33 downward, and thus pushing the exterior bearing surface 42 of the socket against the complementary bearing surface 44 of the valve stem 45. As shown in FIG. 5, the central axis 58 of the socket remains parallel with the central axis 77 of the valve stem 45. Thus, the pivoting motion of the rocker arm 12 is translated to linear and sliding movement of the socket 33 and linear movement of the valve stem 45. Lubrication of the interface 74 limits wear of the contacting bearing surfaces 42, 44. Since the spherical or curved portion 39 of the rocker ball 32 and interior surface 48 of the socket 33 have different shapes, the contact between the rocker ball and the interior surface of the socket is along a segment of the interior surface of the socket and a segment of the spherical portion of the rocker ball. This segment of the socket interior surface is the bearing surface shown at 54 in FIGS. 3-5.
As the valve stem 45 is pushed downward, the valve 47 is unseated from its seat in the head of the engine cylinder 49 (see FIG. 1). The cylinder 49 bears a conventional piston 51 and has a conventional fuel inlet 53. The valve 47 may be either for intake of air or a fuel-air mixture into the cylinder 49 or for exhaust from the cylinder 49.
The ball and socket design is advantageous in that larger bearing surfaces are provided at the interface 74 of the rocker arm and valve stem 45. In addition, the space between the bearing surfaces 42, 44 at the interface 74 allows for an oil film layer to develop between the bearing surfaces 42, 44 to reduce wear. Since the pivoting motion of the rocker arm 12 is translated into sliding and linear motion of the rocker arm socket 33, and thereby into linear movement of the valve stem 45, the flat-against-flat orientation of the socket and valve bearing surfaces 42, 44 is maintained throughout the pivoting motion of the rocker arm 12, maintaining the large surface area of contact.
There are, however, disadvantages associated with the conventional ball and socket design. These disadvantages relate to the problem of retaining the socket 33 on the rocker ball 32 until the rocker assembly 10 is mounted on the engine 15. Generally, to retain the socket 33 on the rocker ball 32, the socket 33 has been designed to extend from below the bottom 40 of the rocker ball up beyond the diameter 76 of the spherical curved part 39 of the rocker ball, and a retainer 66 has been inserted in the interior of the upper end of the socket 33 to retain the socket on the rocker ball. Resilient members, such as O-rings and metal springs have been used as retainers, and have been inserted in a groove 64 in the interior surface 48 of the socket 33. The rocker ball 32 has been pushed past the resilient retainer 66 into position, and the resilient retainer then should return to its shape defining a diameter less than the diameter of the rocker ball 32 to retain the socket on the rocker ball. Metal spring retainers have included flats 68 to define a spacing less than the diameter 76 of the spherical part 39 of the rocker ball 32. These designs have been problematic and expensive: machining the metal slug for the socket 33 to form the depressed interior surface 48 is expensive, and machining the groove 64 in the interior surface 48 of the socket 33 requires a second expensive machining operation. Use of rubber O-rings as retainers has allowed some of the sockets 33 to fall off of the rocker balls 32. Both the O-ring retainers and the metal spring retainers have proven difficult to install in an automated operation. If a metal spring is not properly installed, not only could the socket fall off of the rocker ball, but the metal spring could be deformed in the pressing operation if not properly positioned.
In addition, to provide lubricant to the interface of the socket exterior bearing surface and the valve stem bearing surface, the channels 70, 72 have been machined into the rocker balls and sockets in yet another machining operation. All of these machining operations have added to the cost of the rocker assemblies.